The present invention relates generally to medical imaging and, more particularly, to a system and method of medical imaging having an override capability for a default noise index selection, preferably in computed tomography systems.
Typically, in computed tomography (CT) imaging systems, an X-ray source emits a fan-shaped beam toward an object, such as a patient. The beam, after being attenuated by the patient, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the X-ray beam by the patient. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing unit for analysis which ultimately results in the formation of an image.
Generally, the X-ray source and the detector array are rotated with a gantry within an imaging plane and around the patient. X-ray sources typically include X-ray tubes, which conduct a tube current and emit the X-ray beam at a focal point. X-ray detectors typically include a collimator for collimating X-ray beams received at the detector, a scintillator for converting X-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator.
In one known CT imaging system used to image a volume of interest (VOI), imaging of a patient is conducted by moving the patient through a gantry. Preferably, it is desirable to minimize the patient's exposure to X-rays. To do so, improved signal processing has allowed the use of lower dose CT scans, such as the commercially available 0.5 second CT scanner. Generally, image noise is related to the inverse square root of the dose. However, for different diagnostic objectives and volumes of interest of subjects, higher than needed X-ray exposure doses are known to occur due to the selection of a default noise index which automatically generates a normal default tube current profile. One proposed solution to limit X-ray dosage from the automatically generated tube current profile is to generate an IQ or low dose default tube current profile that automatically generates the default current profile above or below the normal default current profile by a fixed amount. Problems arise, however, when an operator desires to adjust the noise index for the VOI, and in particular, for sub-volumes of the VOI such as when the operator desires to modify X-ray exposure to the subject's sensitive organs, or alternatively when the operator determines that a lower diagnostic quality is sufficient in a sub-volume to achieve the desired patient image.
There is a need for a system that can apply the lowest possible patient dose during imaging by enabling interactive adjustment of an X-ray tube current profile indicative of an applied X-ray dose to the patient. Using existing methods, the setting of a default noise index over the entire VOI may expose a patient to unnecessary X-ray radiation since scanning in certain sub-volumes of the VOI may not require a similar image quality as in other sub-volumes.
Since lower radiation exposure is an on-going goal in X-ray and CT development, it would be desirable to have an imaging system capable of processing pre-scan data to generate a default tube current profile and enabling adjustment of the default tube current profile based on whether a user override is selected.